The present invention relates to a self-supporting terminal for connecting electrode leads to an implantable pulse generator, such as a cardiac pacemaker. The invention further relates to coupling a ceramic to either another ceramic or to a metal via a biocompatible active alloy.
Self-supporting terminals for implantable pulse generators, such as cardiac pacemakers, are generally disclosed in U.S. Pat. No. 4,445,511, which discloses a terminal comprising an alumina ceramic base brazed to a metal terminal piece such as titanium. The ceramic base has a generally cylindrical shape which surrounds the base of the terminal piece. A hole is provided in the upper section of the terminal piece to facilitate the insertion of an electrode lead, which can be fixed in place by means of a screw (such as a grub screw) which is located in the upper section of the terminal piece and disposed so as to project into the path of the lead once it is located in the hole.
The afore-described terminal is subjected to torque from several sources, however. During tightening of the screw to hold the electrode lead in place, for example, a wrench is commonly used which exerts major torque on the terminal. Also, insertion or withdrawal of the electrode lead can result in the application of a sideways, i.e., horizonaal force to the terminal. Any of these forces may be sufficient to destroy the coupling between the ceramic base and terminal piece or pacemaker, thereby destroying the integrity of the terminal.
An important consideration in implantable pulse generators is compactness of the device. As connection terminals are made smaller, alumina ceramics known in the current art of self-supporting terminals lack the necessary strength to withstand the sideways and flexural torque exerted during implantation and adjustment of the pacemaker. To ensure that sufficient strength is provided, severe limitations on the dimensions of the terminal are therefore present. A need is apparent in the art for terminals of sufficient strength yet which have dimensions smaller than the alumina ceramic terminals now in use.
For example, the sideways bending strength S of a connector terminal has been found to be proportional to the cube of the diameter D of a terminal having a cylindrical configuration, i.e., S=D.sup.3. The following table shows the relationship between terminal diameter reduction and required increase in material strength in order to withstand the same sideways load:
______________________________________ Terminal Increase in Material Strength Using Diameter Strength Required Current Material ______________________________________ 1.0 D 0% 1.0 S 0.9 D 37% 0.73 S 0.8 D 95% 0.51 S 0.7 D 192% 0.34 S 0.6 D 363% 0.22 S 0.5 D 700% 0.13 S ______________________________________
Accordingly, presently-used ceramics cannot achieve a sufficient reduction in size or dimension of the connector terminal without the accompanying loss of sideways or flexural strength, as shown above. Specifically, known terminals utilizing alumina ceramics do not have sufficiently high sideways bending or flexural strengths, so suitable reductions in terminal dimensions cannot be achieved.
A further problem associated with the alumina ceramic used in current devices is the possibility of cracking the ceramic during the brazing cycle. In the brazing process, such as shown in U.S. Pat. Nos. 4,426,033 and 4,591,535, the terminal is heated to a brazing temperature followed by a cooling down period. If this cooling down phase is too fast, the alumina ceramic has a tendency to crack due to its low thermal shock resistance. A superior ceramic and brazing alloy is therefore needed in the art of self-supporting terminals.